对比云和降水表征的东亚夏季风活动

利用1998~2007年候平均ISCCP（International Satellite Cloud Climatology Project）D1云资料和台站融合降水资料,定义了两类云指数和降水指数,分别反映东亚夏季风活动期间不同云类云量和降水量位置及强弱的变化。用云指数和降水指数研究了东亚夏季风在中国大陆的推进过程,发现两类指数均能表现东亚夏季风的停滞与北跳特征且具有时空上的一致性。基于云指数变化定义了中国东部华南、华东和华北三个区域季风活跃期、过渡期和中断期,检验了季风活跃期和中断期云指数的差异、500 hPa环流场和水汽场的差异,验证了用云表征季风活动的合理性。对比了用云指数和降水指数定义的季风活动期,发现两个指数定义的季风活跃期和中断期日数虽有差异但基本一致,二者的区别在于降水指数偏重于对降水特征差异的描述,云指数则更偏重于对不同类型云量差异的描述,二者的差异还反映了降水性质的差异。

Comparison of the East Asian Summer Monsoon Activities Represented by Clouds and Precipitation

Based on the pentad average ISCCP (International Satellite Cloud Climatology Project) D1 cloud data and a merged station and satellite precipitation data from 1998 to 2007, two types of cloud index and precipitation index are defined, which respectively reflect changes in clouds and precipitation locations and precipitation intensity associated with the East Asian summer monsoon (EASM) activities. The cloud and precipitation indices are used to study the advancing process of the EASM in mainland China. It is found that both indices can well represent the quasi-stationary stage and the north jump of the EASM, and they are consistent spatially and temporally. Based on changes in the cloud indices, active, transitional and interruptive monsoon periods in three regions of South China, East China and North China are defined. Differences in the cloud indices as well as differences in the circulation and water vapor fields at 500 hPa between the active and interruptive periods are examined, and the reasonability of the cloud indices is proved. Comparison of monsoon activity periods defined by the cloud indices and precipitation index shows that the monsoon active (interruptive) days are basically the same. However, the difference between the cloud and precipitation indices is that the precipitation index emphasizes the change in precipitation, while the cloud indices emphasize the change in cloudiness. The difference between monsoon active (interruptive) days respectively defined by cloud and precipitation indices could also reflect differences in the feature and type of precipitation.